A. Field of the Invention
The present invention relates to a method and an equipment for cooling a continuous casting, and more specifically, to a method for cooling a hot continuous cast steel slab or bloom and equipment for carrying out the same.
B. Description of the Prior Art
Quite recently, with the enhancement of both inner and surface qualities of continuous cast slab, bloom and billet, together with the increase in energy-saving, "direct rolling" in which a hot slab or the like is transferred directly to a rolling step has gained acceptance.
In the conventional method a slab or the like is continuously cast, cut to size, subjected to hot scarfing, cooled, treated for partial hand scarfing to remove surface defect, reheated in a heating furnace, and finally passed through a roughing mill and a finish rolling mill. However, heat loss at the intermediate stages is so great that a considerable amount of heat energy is required for the reheating operation. Therefore a method for eliminating the above reheating furnace was desired. This led to "direct rolling" which was made feasible by improvements over the inner and surface qualities of the continuous casting slab and the fact that treatment by an operator had been made unnecessary.
It is of primary importance that a continuous casting slab and the like should be maintained at as high a temperature as possible, and further, that the whole slab should also be maintained at a uniform temperature, and then, should be delivered to the rolling step in good condition in order to carry out the direct rolling process (referred to "CC-DR process" hereinafter) or a direct charging into a heating furnace and rolling process (referred to "CC-HDC process" hereinafter).
However, the temperature of the continuous casting slab is gradually reduced in the course of transport, and particularly, the temperature of both side edges of the slab falls considerably lower than that of the center of the slab so that the temperature of the slab is inevitably unsuitable for rolling.
One way to overcome the above problem might be to provide an induction heating furnace and a gas heating apparatus at an intermediate stage between the continuous casting equipment and the rolling mill as a means for heating chiefly the side edges of the slab to make the temperature of all portions of the slab as uniform as possible by raising the temperature of the side edges in order to carry out rolling in a smooth manner. However, the above means is not desirable from an energy-saving point of view, and further, it is disadvantageous economically because the equipment cost is high and the electric power required is considerable.
A method for attaining a sound continuous casting slab in which a mist of air-liquid mixture consisting of warm water and a cold medium is sprayed onto the slab to carry out a slow and delayed cooling has been proposed. As compared with water cooling, gas-liquid mist cooling is able to cause a volume of water 200-300 times as large as that of water only to impinge on the slab since the mixed mist splashes three-dimensionally between narrow rolls of the continuous casting machine to achieve uniform sprinkling of the cooling mist so that the cooling ability is exceedingly high.
On the other hand, since the orifice of the spray nozzle used for the gas-liquid mist makes a mist of cold water and air upstream of the spray nozzle, a simple form of slit will suffice for the purpose. Therefore, clogging of the nozzle is no problem at all because the slit can be made to have a larger opening than that of the conventional nozzle by 50-60 times.